This invention relates to electrically operated control systems for controlling operation of gas fired heating apparatus which apparatus includes a main burner ignited by a hot surface igniter and which control system includes a radiant heat sensing switch.
U.S. Pat. No. 5,169,301, assigned to the assignee of the present invention, discloses a control system for a gas fired heating apparatus which apparatus includes a main burner, two gas valves connected fluidically in series with the main burner and each gas valve having a controlling electrical winding, and a hot surface igniter for directly igniting gas at the main burner, and which control system includes a radiant heat sensing switch responsive to the igniter and burner flame for controlling the energizing of the igniter and the electrical windings of the gas valves. The radiant heat sensing switch therein has a single set of contacts, normally-closed, which open in response to the igniter being at a temperature above gas ignition temperature and are maintained open thereafter in response to burner flame. The radiant heat sensing switch therein, when its contacts are closed, provides an energizing circuit through its contacts to a relay coil which controls a set of normally-closed contacts and a set of normally-open contacts. When the relay is energized, its normally-open contacts close, enabling energizing of the electrical winding of one of the gas valves, and its normally-closed contacts open, preventing energizing of the electrical winding of the other gas valve. When the radiant heat sensing switch contacts open, the relay is de-energized. With the relay de-energized, the electrical winding of the one gas valve is maintained energized through a circuit in parallel with the relay's normally-open contacts, and the electrical winding of the other gas valve is energized through the relay's normally-closed contacts.